Tracing fluid sources, their influence, and mechanical consequences in the Inuyama accretionary complex, Japan

Vein stable isotope geochemistry and minerology provide a powerful record of fluid sources in subduction-zone fault systems, with implications for fault mechanics and seismic behaviour. We investigate fluid sources along out-of-sequence thrusts within an exceptionally well-preserved exhumed analogue of the shallow seismogenic zone: the Inuyama Sequence from the Jurassic Accretionary Complex in central Japan. The sequence comprises a coherent ocean-floor stratigraphy of siliceous claystone, ribbon chert, siliceous mudstone and clastic units, repeated by thrust imbrication.

Vein and host rock stable isotope data reveal the presence of two distinct vein sets, implying two distinct fluids, within the thrust sheets. One fluid is a cool pore water (𝛿¹⁸O = - 4 to 0 ‰), that precipitated quartz (21.2 to 25.7 ‰), calcite (20.6 to 21.1 ‰), and rhodochrosite (25.2 ‰) veins at ∼40 to100 ˚C throughout the exposed thrust sheets. This is consistent with a seawater-derived pore fluid in the shallow accretionary prism. In contrast, some quartz (2.1 ‰) and calcite (-0.4 to 6.2 ‰) vein clusters require a different and warmer fluid (𝛿¹⁸O = - 11 to - 8 ‰) possibly of meteoric origin. These isotopically lighter veins are restricted to discrete shear zones with well-developed scaly fabric and are generally focused along the margins of mechanically competent blocks. Notably, these discrete shear zones have far higher carbon contents than the host rocks, be that through pressure solution or direct carbon precipitation. The isotopically lighter calcite and quartz veins record significantly higher temperatures (∼170 to 220 ˚C), confirmed with chlorite geothermometry, and are in line with Raman and vitrinite reflectance temperature estimates for peak conditions for the area (Kameda et al., 2012; Ujiie et al., 2021).

The occurrence of isotopically light fluids at temperatures of 170-220˚C, corresponding to depths of ~ 8.5 to11 km given a relatively cool accretionary geotherm (20 ˚C/km), requires either (1) deep and lateral ingress of meteoric waters into the inner wedge as accreted sediments approached the coast, (2) late-stage vein precipitation during exhumation and fault reactivation, or (3) kinetic isotope effects associated with rapid precipitation during fault dilation that drives 𝛿¹⁸O lower than those predicted for equilibrium precipitation. Importantly, the hot and isotopically light fluids show a strong spatial relationship with highly concentrated black carbonaceous material that appears to control strain localisation at thrust sheet contacts. Consequently, these fluid-driven mechanical changes may have created carbon-rich asperities of very low frictional strength, encouraging local aseismic creep and stress build up at asperity boundaries.